1. Field of the Invention
This invention relates to radar technology, and more particularly, to a method for use on a coherent Doppler radar system for the purpose of reducing clutter and mutual interference in the coherent Doppler radar system.
2. Description of Related Art
Mutual interference would occur in a coherent Doppler radar system if it is installed near or beside another radar system of the same type or operating substantially at the same frequency. If the amplitude of mutual interference exceeds the tolerable noise level of the radar system, it would adversely affect the radar operation.
FIGS. 1A-1C are schematic diagrams used to depict three different forms of mutual interference; wherein FIG. 1A shows the case of mutual interference between two major lobes; FIG. 1B shows the case of mutual interference between major lobe and minor lobes; and FIG. 1C shows the case of mutual interference between the minor lobes of one source and the minor lobes of another source. Since major lobe possesses greater energy than minor lobes, the case of FIG. 1A would cause the greatest level of mutual interference as compared to the other two cases. When mutual interference occurs, it would affect the accuracy of range detection and target tracking by the radar system.
It is therefore an objective of this invention to provide a method for use on a coherent Doppler radar system for the purpose of reducing clutter and mutual interference in the coherent Doppler radar system.
The method of the invention is characterized in the use of a Crisscrossing CFAR (Constant False Alarm Ratio) method for computing the CFAR background value in the event of the presence of mutual interference
The invention is particularly useful for eliminating mutual interference resulted from a nearby radar system operating at a close or the same frequency, allowing the detected radar data about the target object to be more accurately displayed.
The radar system utilizing the method of the invention comprises the following system parts: (a) a transmitter for generating a pulsed radar signal composed of a series of pulses spaced at a predetermined pulse repetition interval PRI; (b) an antenna for emitting the pulsed radar signal from the transmitter into the ether, and in the event of the emitted radar signal meets the target object, picking up the echoed radar signal; (c) a receiver for receiving the echoed radar signal picked up by the antenna and performing a preliminary signal processing procedure on the echoed radar signal; (d) an analog processor, coupled to the receiver, for processing the output signal of the receiver and thereby generating a video frequency signal; (e) a digital signal processor coupled to the analog processor, which is capable of digitally filtering out the clutter and mutual interference in the output video frequency signal from the analog processor to thereby generate a radar display signal; and (f) a radar display unit, which is driven by the radar display signal from the digital signal processor, for displaying the radar information about the target object.
The method of the invention comprises the following steps: (1) performing a coherent processing procedure on the echoed radar signal to thereby convert the echoed radar signal into a Range-Doppler Domain representation consisting of a number of range cells and a number of Doppler cells; (2) from the Range-Doppler Domain, determining a mutual interference detector (MID) Range Crossing value and a MID Doppler Crossing value; (3) from the MID Range Crossing value and the MID Doppler Crossing value, computing for the value of MID Range Crossing Repeat Interval and the value of MID Doppler Crossing Repeat Interval; (4) checking whether the MID Range Crossing Repeat Interval is equal to the MID Doppler Crossing Repeat Interval; (5) if YES, performing a Crisscrossing CFAR method to compute for the value of CFAR background; (6) selecting a cell under test (CUT) from the Range-Doppler Domain; (7) computing the amplitude of the selected CUT against the CFAR background; (8) specifying a minimum threshold, a CFAR background multiple, and a PRF Index; (9) computing for the threshold of the selected CUT, where the threshold of the selected CUT is equal to the multiplication of the CFAR background value of the selected CUT by the CFAR background multiple; (10) checking whether the amplitude of the selected CUT is greater than or equal to the threshold of the selected CUT and also greater than the minimum threshold; (11) if YES, indicating that the selected CUT corresponds to the target object and outputting the amplitude of the selected CUT together with the PRF Index to a radar display unit; and (12) based on the amplitude of the selected CUT and the PRF Index, displaying the location and velocity of the target object on the radar display unit.